As used herein, the term "fibers" or "fiber" refers both to filaments (strands of indefinite or continuous length) and staple (strands of short and definite length).
There are several general methods for extruding thermoplastic polymers into fibers. One of these methods is known as grid spinning because the solid flakes or chips of polymer are melted on a heated grid. U.S. Pat. No. 2,683,073 to Pierce and U.S. Pat. No. 3,102,301 to Dechene describe conventional grid spinning processes.
In the manufacture of fibers, it is sometimes desirable to introduce different materials into the melt in order to provide the fibers with certain functional characteristics. For example, TiO.sub.2 is sometimes added to the polymer melt to deluster the resulting fiber. There are several methods for adding such additives to the grid spinning fiber extrusion process. The solid polymer chips or flakes may be dusted with the additive prior to melting the chips or flakes on the grid. One such dusting process is described in U.S. Pat. No. 4,490,542 to Iqbal et at. Similarly, U.S. Pat. No. 4,379,913 to Waitkus describes adding antistatic agents from a solvent solution to chips or flakes of polymer, drying the coated polymer chips or flakes and spinning the dried coated polymer chips or flakes to yam by, for example, melting on a grid into a stirred pool. U.S. Pat. No. 3,923,727 to Jost et al. describes sprinkling chips with finely divided dye stuff and then melting the chips according to usual grid spinning processes.
Another method for introducing additives is to mix solid additive concentrate pellets with the host polymer chips or flakes. U.S. Pat. No. 3,829,543 to Robertson describes a process for mixing pellets of a coloring additive with chips of the host polymer prior to melting.
Incidentally, another method for melt spinning is to melt the polymer chips in a screw extruder. Additives may be added directly to the extruder, often using a sidearm of the extruder. U.S. Pat. No. 4,405,734 to Fuchs et al. describes a process wherein dye stuff can be metered directly into the melt of the thermoplastic via a side screw to achieve homogeneous mixing or using a coiled grid spinning system. Generally, screw extrusion processes result in fairly homogeneous mixtures of the additives in the melt due to the mixing action of the screw. Grid melting systems suffer in this regard because the melting process itself is without agitation.
The lack of agitation has been recognized to cause certain problems such as gel formation and spherulite seeding due to unmelted particles. Equipping the grid with mechanical stirring is one method of providing homogeneity in the melt. U.S. Pat. No. 4,453,867 to Sharps describes a disc stirring apparatus for grid melting systems. U.S. Pat. No. 4,639,205 to Lim describes a shaft impeller for grid melting systems.
U.S. Pat. No. 5,157,067 to Burditt et al. describes a liquid concentrate system that may be incorporated into fibers. U.S. Pat. No. 5,364,582 to Lilly describes the use of the liquid concentrate described in U.S. Pat. No. 5,157,067 to introduce antistatic agents into the throat of a fiber spinning extruder.
Not all grid spinning equipment is, however, equipped with agitation means. There remains a need for homogeneously introducing additives into generally unagitated grid melting systems.